The separation of sodium sulfate (salt cake) crystals from a mother liquor obtained during the production of chlorine dioxide has been disclosed in patents such as U.S. Pat. No. 4,086,329 entitled INTEGRATED CHLORINE DIOXIDE PRODUCING SYSTEM, Inventors: Gerald Cowley et al. The present invention further purifies such salt cake crystals. The sodium sulfate crystals (22) obtained from separator 20 shown in FIG. 1 of U.S. Pat. No. 4,086,329 contains substantial quantities of chloride ions associated with the crystals. Because of the presence of the chloride ions and because of the large particle size of the crystals, the impure crystals may not be suitable for many purposes. In particular, due to the chloride and chlorate contamination, the sodium sulfate crystals should not be used as a source of sodium sulfate in the makeup of pulping liquor in a kraft pulping process because the chloride ions would introduce substantial corrosion problems particularly in the recovery boilers of conventional kraft pulping processes.